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Cameron Reid,17, San Anselmo escapes the heat as he goes for swimming hole plunge in the Paper Mill Creek near Samuel P. Taylor State Park in Marin County. Event on 7/11/05 in Marin County Eric Luse / The Chronicle MANDATORY CREDIT FOR PHOTOG AND SF CHRONICLE/ -MAGS OUT less

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Cameron Reid,17, San Anselmo escapes the heat as he goes for swimming hole plunge in the Paper Mill Creek near Samuel P. Taylor State Park in Marin County. Event on 7/11/05 in Marin County ... more

Photo: Eric Luse

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The fountains at the Broadway Plaza in Walnut Creek got lots of use Monday as temperatures soared. Sixteen month old Anastasia Cedarborg smiled as she watched her brother Gabriel demonstrate his kicking technique. Summer finally came to Contra Costa County Monday as temperatures seemed a little hot. The rest of the week should be even a bit warmerBrant Ward 7/12/05 less

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The fountains at the Broadway Plaza in Walnut Creek got lots of use Monday as temperatures soared. Sixteen month old Anastasia Cedarborg smiled as she watched her brother Gabriel ... more

Photo: Brant Ward

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On 7/11/05 in San Francisco with clear blue skies and temperatures in the 70's, (L TO R) Elenor Caurera couldn't help taking her lunch break "barefoot in the park" in Civic center plaza Monday while in the background Ryan Brooks relaxes shirtless after taking a run.
Kat Wade/ The Chronicle MANDATORY CREDIT FOR PHOTOG AND SF CHRONICLE/ -MAGS OUT less

HOT13_015_KW.jpg
On 7/11/05 in San Francisco with clear blue skies and temperatures in the 70's, (L TO R) Elenor Caurera couldn't help taking her lunch break "barefoot in the park" in Civic center plaza Monday ... more

Photo: Kat Wade

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BAY AREA / No sweat! Summer finally heats up / Break out those kiddie pools for highest temperatures of year

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It's gonna be a scorcher today -- a fry-an-egg-on-the-sidewalk, hot-enough-for-you? kind of day where the mercury will hit the century mark and stay there awhile.

After getting a late start, summer is expected to make up for lost time by socking the Bay Area with blistering heat and the highest temperatures of the year.

"Most of the weather we've seen has been a few weeks behind schedule," said Steve Anderson, a forecaster with the National Weather Service. "Usually we start seeing the hot temperatures in late June."

Those living in inland areas -- the oven that is central Contra Costa County and points east -- will break out the kiddie pools and strain their air conditioners seeking solace from triple-digit heat. Even folks along the coast may break a sweat today -- after all, the high 70s are positively scorching when you live in Pacifica.

Although it will be hot enough to justify an ice cream cone at lunch and a cold one after work, it won't be hot enough to score a free ride to work this morning.

The atmosphere isn't still enough to allow pollutants to settle in, so it won't be a Spare the Air Day. That's the call of the Bay Area Air Quality Management District which, along with the Metropolitan Transit Commission, will start paying for free morning rides for commuters aboard the region's 21 mass transit systems when the smog hits full force.

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"We're watching Wednesday very carefully," said Terry Lee, a spokesman for the air quality district. "We're ready to trigger it."

Temperatures have already started to climb. Monday's highs included 92 in Santa Rosa, 91 in Concord and Livermore, and 85 in Redwood City.

The departure from weeks of lovely weather is due to the arrival of a dominant ridge of high pressure, which is creating a warm air mass that will envelop the region, Anderson said.

Translated from meteorologese, that means Arizona is sending its hot air our way.

"It's like a big bubble of warm air, and it's going to gradually slide over the Bay Area from the desert Southwest," Anderson said.

That bubble will be biggest over the inland areas, meaning residents of inland areas of Monterey and San Benito counties will be subjected to temperatures nearing 110 degrees. Folks along the Interstate 680 corridor, the eastern stretches of Contra Costa County and the Napa Valley won't have it much better as they swelter in 100-degree heat.

San Jose and the rest of the South Bay will hit the 90s, while people in San Francisco and along the coast will see more reasonable temperatures in the high 70s.

But never fear -- cooler temperatures will soon be here.

"It'll level off or even be cool around the Bay Area on Thursday and be back to normal next week," said Mike Pechner, owner of Golden West Meteorology. We'll enjoy a couple of weeks of normal temperatures, but swelter again at the end of the month, he said.

"The long-range data show very hot weather during the last week of July and the first week of August," Pechner said, adding that San Francisco might see the mercury top 90.